1. Field of the Invention
The present invention relates to glass powders having well controlled chemical and mechanical properties as well as methods for producing glass powders, and intermediate products and devices incorporating the glass powders. The glass powders are preferably produced by spray pyrolysis of glass precursors.
2. Description of Related Art
Many product applications require glass powders that have one or more of the following properties: spherical morphology; high purity; small average size; narrow size distribution; controlled chemistry; and little or no agglomeration. Examples of glass powder applications requiring such characteristics include, but are not limited to, thick-film pastes used for fabricating electronic devices. Thick-film pastes are mixtures of fine powders in an organic vehicle, wherein the organic vehicle is removed after application of the paste to a substrate.
In particular, many product applications require glass powders having a small average size, such as from about 0.1 μm to about 10 μm, and a spherical morphology. It can also be advantageous if the powder consists of glass particles having a narrow size distribution without any substantial agglomeration of the particles. Most glass powders are produced by forming a melt of the desired glass composition, quenching the molten glass and milling the resulting glass to reduce the particle size of the glass. See, for example, U.S. Pat. No. 4,820,661 by Nair which discloses an aluminoborosilicate glass useful in thick-film compositions for crossover dielectrics. Such methods result in glass powders having a jagged and irregular morphology, wide spread of particle size and other characteristics which are undesirable in precision applications.
There have been attempts in the art to produce glass particles having improved chemical and physical characteristics. U.S. Pat. No. 4,775,520 by Unger et al. discloses a process for forming monodispersed silica (SiO2) particles having an average size of from about 0.05 to about 10 μm. The particles, which are useful as sorption materials in chromatography, are formed by a two-step process including hydrolytic polycondensation and the addition of a silane to control the reaction.
U.S. Pat. No. 5,173,457 by Shorthouse discloses a borosilicate glass useful for thick-film applications having a size of less than 5 μm and a spherical morphology. The glass is formed by a sol-gel process.
U.S. Pat. No. 5,589,150 by Kano et al. discloses a silica gel formed by milling a hydrogel slurry and spray drying the slurry to form gel particles having an average size of 30 to 100 μm and a spherical morphology. The gel particles are useful as polymer catalyst carriers.
Typically, sol-gel and related precipitation routes for forming glasses are limited in their usefulness. The precursors, such as alkoxides, are prohibitively expensive and the process cannot easily be converted to a continuous production method. It is also difficult to produce complex glass compositions with good homogeneity of the different glass components. Further, it can be difficult to separate the glass particles from the liquid in which they are produced.
The continued miniaturization and increased complexity of electronic components has created a need for materials, including glass particles, with well-controlled physical and chemical characteristics. For example, thick-film paste technology must continue to meet the demands of decreased line width and decreased pitch, i.e., decreased distance between traces. As a result, pastes have been developed that have a photo-imaging capability to enable the formation of traces having a decreased width and pitch. In this process, a photoactive thick-film paste is applied to a substrate and the paste is then dried and exposed to ultraviolet light through a photomask and the exposed portions of the paste are developed to remove unwanted portions of the paste.
Examples of such photoactive pastes are disclosed in: U.S. Pat. No. 3,958,996 by Inskip; U.S. Pat. No. 4,119,480 by Nishi et al.; U.S. Pat. No. 4,598,037 by Felten; U.S. Pat. No. 4,613,560 by Dueber et al.; and U.S. Pat. Nos. 5,032,478 and 5,032,490 both by Nebe et al. Each of the foregoing U.S. Patents are incorporated herein by reference in their entirety.
Glass compositions for thick-film pastes are also useful for forming dielectric layers in electronic circuits. For example, U.S. Pat. No. 4,820,661 by Nair discloses an aluminoborosilcate glass useful for crossover dielectrics. U.S. Pat. No. 4,959,330 by Donohue et al. discloses a crystallizable glass including ZnO and BaO that is useful as a dielectric layer. U.S. Pat. No. 5,210,057 by Haun et al. discloses an alkaline earth zinc silicate glass that is partially crystallizable and is useful for forming dielectric layers. Each of the foregoing U.S. patents are incorporated herein by reference in their entirety.
To meet the demands of these and similar applications, glass powders, particularly complex glass powders, having well-controlled physical and chemical characteristics are required. To date, such glass powders have not been provided.
It would be particularly advantageous to provide a flexible production method capable of producing glass powders which would enable control over the powder characteristics as well as the versatility to accommodate complex glass compositions which are either difficult or impossible to produce using existing production methods. For example, it would be advantageous to provide control over the particle size, particle size distribution, morphology, homogeneity, and porosity of the glass powder. It would be particularly advantageous if such glass powders could be produced in large quantities on a substantially continuous basis.